build: pin pytest to < 7

It seems like pytest 7 breaks pytest-forked somehow, as also reported in
pytest-dev/pytest#9621

Hence, for now pin it to any version lower than 7 to keep the unit tests
working.

(Cherry-picked from f801b69)

Author: ptoscano

test-requirements.txt

@@ -7,7 +7,7 @@
 -e ./syspurpose
 
 flake8<4
-pytest
+pytest<7
 pytest-randomly
 pytest-timeout
 pytest-forked